This invention is a device for improving the effectiveness of and reducing the cost of any process of the type known to the art as reactive sputtering when the process requires that a low resistance conducting path for the target power supply be maintained for its duration, and when an insulating film on the substrate is created by the process. The target power supply may be a DC supply or it may be an AC supply which operates at a frequency that is too low to allow capacitive coupling to provide a current return to ground.
DC or low frequency AC sputtering is often preferable to sputtering using an RF supply (RF sputtering). Higher sputtering rates are achievable and power supplies are simpler as well as cheaper. RF sputtering presents difficulties when large targets are employed so that the electromagnetic wavelength at the frequency of operation (normally 13.56 MHz) becomes comparable to the size of the target. In such cases standing waves cause hot spots and instabilities leading to coating inhomogeneity and nonuniformity. Standing waves are, of course, not present in DC or low frequency AC sputtering.
As practiced by the prior art, however, reactive DC or low frequency AC sputtering processes have been subject to instability caused by a phenomenon which has become known to the art as the xe2x80x9cdisappearing anodexe2x80x9d. For reasons which will now be stated, the present invention prevents the disappearing anode phenomenon from occurring, thereby enhancing the stability of the processes.
A typical DC sputtering apparatus consists of a sputtering target located in a vacuum vessel along with a substrate upon which sputtered material is to be deposited. The atmosphere in the chamber contains a sputtering gas, usually argon, at an appropriately low pressure. When the process is operating, the sputtering power supply is connected between the target and the chamber walls causing a plasma to be formed in the chamber. The target is the cathode. In a region adjacent to the target called the sheath, a voltage drop that is, in the absence of a voltage drop in the plasma, roughly equal to the power supply voltage (typically 500 volts) occurs, and in this region ions of the sputtering gas are accelerated toward the target. When these ions strike the target they cause the metal which forms the target, such as silicon or titanium, to be sputtered from the target surface into the space between the target and the substrate. The atoms then travel through this space on trajectories which may involve many collisions until they strike some surface and become attached to it. A buildup of sputtered material on the substrate and on other surfaces in the vicinity of the target and substrate begins. The (non-insulating) sputtered material then reacts with a gas such as oxygen that has been introduced into the chamber, forming the coating that is desired for the substrate, which is an insulating material such as silicon dioxide or titanium dioxide, on all surfaces which have been coated with sputtered material.
In some reactive sputtering processes, the sputtering gas and the reactive gas are the same. Such processes function in the same way as has just been described, except that since only one species of gas such as oxygen is introduced into the chamber, ions of this species sputter metal from the target, and activated forms of the same species subsequently react with the sputtered metal on the substrate surface.
The positively charged ions that strike the target bring about a net flow of positive charge from the plasma in the chamber to the target. This flow of charge to the target must be balanced by an equal transfer of charge by electrons traveling from the plasma to the chamber walls or other grounded conductors to form the return path for the target power supply current. As is well known, under conditions of low current, the bulk plasma exists in a region of relatively constant potential, called the plasma potential. In the presence of electric current, there is a potential gradient in the bulk plasma which is proportional to the current density and inversely proportional to the density of charge carriers at each point in the plasma.
The potential gradient, measured in volts/cm, results in a voltage drop across the bulk plasma between the target and the chamber walls, and power is dissipated within the plasma in proportion to the product of the target current by the voltage drop in the plasma. This power is obtained at the expense of power being consumed in sustaining the intense ionization at the target. If the plasma is physically extended, voltage gradients within it are insufficient to cause a compensating ionization in the bulk plasma. Therefore, a voltage drop within the plasma decreases the amount of power supply energy that is available for ionization and consequently results in a decrease in electron density within the plasma. Since the density decrease causes a further increase in the voltage drop, the process tends to go out of control when the plasma density in the region through which the electric current must pass becomes low enough to cause the voltage drop across the plasma to become a significant portion of the power supply voltage. Under such conditions instabilities or complete plasma shutdown may occur.
An unacceptably large voltage drop within the plasma inevitably occurs in a reactive DC or low frequency reactive sputtering process due to restriction of the current flow by the progressive formation of insulating material on the conductors near the target. An initial buildup of insulating material on the conducting surfaces near the target causes the area of the walls available for electron current flow to decrease and to move farther away from the target. The region of plasma in which current must flow to find a path to ground therefore moves further away from the target so as to contact uncoated sections of wall. This forces the current to take a longer path through a region which is farther away from the region of plasma generation and where the plasma is consequently less dense. Furthermore, blockage of the current path reduces the area of the chamber walls which is accessible to the return current, so that the cross sectional area of the current flow is reduced, and, if the ion current to the target is kept constant, the current density increases. The voltage gradient in the bulk plasma, which is proportional to the current density and inversely proportional to the plasma density, is increased by both these factors. This increase in the gradient eventually brings about a voltage drop across the plasma which is a substantial fraction of the supply voltage causing the above-mentioned instability or shutdown of the plasma.
In most DC or low frequency AC sputtering processes as practice by the current art, a magnetic field is introduced into the region of the plasma that is adjacent the target. The rate of generation of ions of the sputtering gas in the region above the target is thereby increased over that which would be achieved without the magnetic field, which then brings about a corresponding increase in sputtering rate and the sputtering plasma which must pass through the plasma. Processes which employ such magnetic fields are called magnetron sputtering processes. The adverse affects of the oxide buildup which are mitigated by the present invention exist whether or not the process is a magnetron process.
The effect of the voltage drop across the plasma can be compensated to some extent by continuously adjusting the voltage of the target power supply so as to maintain a constant target current for the duration of the process. This approach has the disadvantages that a power supply of considerably greater capacity and complexity is required and that a considerable portion of the power is dissipated in the gas within the chamber, wasting power and causing undesirable heating.
For the above reasons the buildup of insulator and corresponding voltage drop across the plasma during the sputtering process requires that the process be interrupted after a certain amount of material has been deposited to allow cleaning or replacement of parts that are close to the sputtering target. The result is that such processes are more costly and provide lower rates of production of coated parts than a process in which such cleaning or replacement would not be required.
It is therefore an objective of this invention to provide an environment within the sputtering chamber during a DC or low frequency AC sputtering process in which a relatively constant low resistance current path to ground is maintained for a period of time whose duration greatly exceeds that provided by current art.
It is a further objective of this invention to improve the stability of such a process by providing a source of plasma within a sputtering chamber that is separate from the target power supply and which therefore can be maintained in a spatially uniform condition throughout a large volume of the chamber for the duration of the process while allowing the target voltage and current to remain fixed.
The invention is a device for performing an operation known to the art as reactive DC sputtering or reactive low frequency AC sputtering. It may be employed in a batch process which employs a rotating apparatus such as a drum within a chamber or in a continuous process such as a roll coater or in-line coater. At least one wide area plasma applicator produces a plasma having a required electron density and uniformity within a region such that a current can flow through the plasma from the sputtering target to portions of the grounded walls of the sputtering chamber that are remote from the target.
The plasma generated by the applicator diffuses into the region where sputtering is taking place where it commingles with the plasma of the sputtering process. The plasma also diffuses into regions which are remote from the sputtering region where it makes contact with the walls of the sputtering chamber and with other grounded conductors. In these remote regions there is no oxide formation so that electron current can freely flow to the walls or other conducting objects. The applicator causes a current path through the combined plasmas from the target to the chamber walls to exist, so that the chamber walls or other conducting objects remote from the sputtering process and not subject to insulator formation can serve as the anode. Thus the invention assures the presence of a reliable current path to ground during the sputtering process.
The portion of the plasma which commingles with the plasma generated by the DC target potential causes the electron temperature and the ion and electron density in the sputtering region to attain a level that is higher than the level which they would reach if the applicator were not present. The aforesaid higher densities bring about a corresponding enrichment of the densities of reactive species at the surface of the substrate. The density of reactive species remains stable and high during the entire process and is not influenced by the formation of insulating material on nearby surfaces. Thus control of the reaction process at the substrate surface is provided throughout the process.